The present invention relates to a method of manufacturing a semiconductor device and an apparatus for manufacturing a semiconductor device, particularly, to a method and apparatus for forming, for example, a gate insulating film included in a MOS transistor.
A magnetic disk memory device is widely used as a memory device included in an information processing apparatus. However, the magnetic disk memory device requires a highly precise mechanical driving mechanism and, thus, is weak against impact. Further, in the magnetic disk memory device, a magnetic head or a memory medium must be mechanically operated in recording information onto the memory medium or in reading information from the memory medium. As a result, it is impossible to achieve a high speed access in the magnetic disk memory device.
On the other hand, a semiconductor memory device does not include a portion that must be mechanically driven. Therefore, the semiconductor memory device is strong against impact and a high speed access can be achieved. Such being the situation, demands for a semiconductor memory device are on a sharp increase in recent years, and vigorous researches are being made on the semiconductor memory device.
The research and development of the semiconductor memory device are being directed mainly to miniaturization of the memory cell by making progress in the fine processing technology. Specifically, it is intended to improve the memory capacity, etc. by proportionally diminishing the memory cell. Naturally, the miniaturization of the memory cell necessitates reduction in the thickness of the gate insulating film. However, reduction in the thickness of the gate insulation film gives rise to problems relating to reliability of the gate insulating film and to performance of the transistor.
For example, in a nonvolatile memory in which a memory cell is formed of a transistor having a thin gate insulating film through which flows a tunnel current, i.e., a tunnel insulating film, the thickness of the tunnel insulating film tends to be decreased in accordance with improvement in the degree of integration.
However, with decease in the thickness of the tunnel insulating film, since a high electric field stress is applied to the tunnel insulating film, a serious problem is brought about that a leak current flowing in a region of a low electric field, i.e., a stress leak current, is increased. Also worried about is, for example, the lowering of Q.sub.bd characteristics, i.e., increase in the trapped electrons and decrease in the amount of charge that can be allowed to flow through the insulating film.
For overcoming the above-noted problems, a silicon oxynitride film is used as the tunnel insulating film in place of a silicon oxide film. The silicon oxynitride film differs from the silicon oxide film in that a weak Si--O bond is modified by a nitrogen atom. Therefore, the use of an oxynitride film as a tunnel insulating film is considered to be effective for suppressing the generation of the electron trapping caused by the application of a high electric field stress and for suppressing an increase in the stress current. However, satisfactory characteristics cannot be obtained in the case of using the oxynitride film formed by the conventional method.
For example, the oxynitride film used as the tunnel insulating film can be formed by forming first a silicon oxide film on a surface of a silicon wafer, followed by nitriding the silicon oxide film by using an ammonia (NH.sub.3) gas and subsequently annealing the film by using an oxygen gas. This method makes it possible to introduce nitrogen atoms that are important for enhancing the reliability of the oxynitride film into the entire region of the film with a high concentration.
In this method, however, hydrogen is also introduced into the film during the nitriding process for introducing nitrogen atoms into the film. Although, the introduced hydrogen atoms are mostly liberated during the subsequent oxidizing process, some of the hydrogen atoms remain unremoved within the film. The hydrogen atoms remaining within the film bring about insulation breakdown, formation of electron trap, and increase in the stress leak current. Such being the situation, it is desirable to employ a process for forming an oxynitride film under a hydrogen-free condition in place the process using an ammonia gas.
A process using an N.sub.2 O gas and a process using both an NO gas and an O.sub.2 gas are known to the art as a process for forming an oxynitride film under a hydrogen-free condition. In the process using an N.sub.2 O gas, an N.sub.2 O gas is supplied onto a surface of a silicon wafer heated to, for example, about 950.degree. C. The N.sub.2 O gas arriving at a region in the vicinity of the wafer surface is heated so as to be decomposed into a nitrogen gas (N.sub.2), an oxygen gas (O.sub.2) and nitrogen monoxide gas (NO). Where the wafer temperature is set at, for example, 950.degree. C., the N.sub.2 O gas is decomposed into 64.3% of N.sub.2 gas, 31.3% of O.sub.2 gas and 4.7% of NO gas. In this process, the oxidizing species of O.sub.2 and the nitriding species of NO, which are generated by the thermal decomposition of N.sub.2 O, are involved in the formation of an oxynitride film. In this method, however, the nitriding species of NO gas is formed only slightly, resulting in failure to introduce a sufficiently large amount of nitrogen atoms into the film.
On the other hand, in the process using an NO gas and an O.sub.2 gas, the NO gas and the O.sub.2 gas are supplied onto a heated surface of the wafer so as to form an oxynitride film. In this method, it is considered possible to supply a sufficiently large amount of an NO gas unlike the process using an N.sub.2 O gas. In fact, however, a gaseous phase reaction given below takes place before the NO gas and the O.sub.2 gas arrive at the wafer surface: EQU 2NO+O.sub.2.fwdarw.2NO.sub.2
As a result, a large proportion of the NO gas is consumed for formation of the NO.sub.2 gas that is not involved in the nitriding reaction on the surface of the wafer, leading to a low NO concentration in the vicinity of the wafer surface. It follows that it is difficult to increase the nitrogen concentration in the film in the known process using N.sub.2 O gas or both NO gas and O.sub.2 gas.